Treating silicon steel



which gested for ordinar questionable whet of annealing would Patented Jan. 19, 1932 UNETED STATES PATENT OFFICE CARL E. GIITORD, OI HIDDLETOWN,

OHIO, ASSIGNOR TO THE AMERICAN ROLLING MILL TREATIN G SILICON STEEL 80 Drawing.

My invention relates to the production of silicon steel for electrical purposes, and is particularly directed to the obtaining of improved surface conditions in the product after it has been annealed to impart proper electrical properties.

The treatment of silicon steel sheets in order to clean and flatten them, and anneal them without injuring the surface, is quite difficult, due to a number of factors among which are the adhesiveness of the mill scale on the hot rolled sheet, which calls for particular care in the pickling operation, and the troublesome tendency of the sheets to stick to each other during the annealing cycle.

It has been the practice to underpickle silicon.

steel sheets so as to avoid pickle brittleness and to leave some of the scale in place, due. to the tendenc of perfectl stick to-each other. It has afs tice to use a mechanicalse lime in order to prevent t sheets during annealing. methods are to v t is the object of my invention to provide a separator for silicon steel sheets during annealing, which undergoes a chemical reaction in COIIJllIlCtiOIl with the silicon in the steel, and resultsin the formation of a separator 0 been the pracarator such as e'" adhesion of Both of these not, however, entirely satisfacor spacer.

I accomplish my object by providing the sheets wit a coating of oxide, either prepared on the sheet itself, or imposed "upon the sheet, and I will describe a number of different ways of obtaining this as compared, for example with the oxide will form naturally on the roduct upon exposure to the air within t e mill buildings.

The idea of using a thin film of oxide as a separator during annealing has been sugsteel sheets, but it is very or such films would serve The reducing conditions reduce the oxide content of the film and the ferrous iron thus produced would be likely to bind or weld the sheets together, instead of spacing'them.

any good purpose.

clean sheets to oxide coat-' ing which must be of substantial thickness,

Application filed February 11, 1929. Serial No. 339,279.

With a fairly thick oxide coating on silicon steel, however, the result of the reducing conditions of annealing is to form ferrous iron, as above indicated, but a portion of the oxygen combines with the silicon in the steel producingsilica, which forms a fine whitish powder, nonadherent, and readily removed by mechanical means. This silica is the product which functions as the spacer.

The probable chemical reaction which 1takes place during the annealing is as 'folows:

Reduction 2Fe O =4Fe 30 i Oxidation 30 2Si =2Si02 02 According to one process which I have followed with success, hot rolled silicon steel sheets are pickled in sulphuric acid solution, to an extent which entirely removes all scale thereon, leaving them perfectly clean.

The sheets are then washed so as to be free from acid, and passed slowly through a furnace under oxidizing conditions. In the furnace a temperature of around 1200 degrees Fahrenheit may be maintained, and the time of exposure within the furnace should be such as to build up a thick coating of oxide.

Such ickle brittleness as develops -be cause of ully pickling the sheets, is dissipated by the heating thereof when oxidizing them. A temperature of around 1200 degrees is'not such as will effect the electrical properties under the conditions noted.

The sheets with the oxide coating hereon, and formed into piles, are placed into annealing boxes and given the annealing treatment in the usual manner for imparting desired electrical properties. If desired, additional spacer material such as lime may be employed, although in my present practice, I do not find any advantage in doing so.

If required to impart flatness, there may be a cold rolling between the several beatings of the annealing cycle.

If the oxide coating is thick enough, the result of the annealing is to bring about the chemical reactions above indicated, and the product after annealing is covered with a fine whitish powder which is mainly formed of silica. This silica is readily brushed, blasted or wiped awa leaving the sheets with a fine silvery sur ace of great sm0othness and uniformity.

There will be practically no sticking of sheets to each other due to the silica spacer, the effectiveness of the spacer being a parently directly proportional to the t ic ness of the primar oxide coating.

It will be ound that silicon steel sheets will develop the maximum oxide coating, due to heating under oxidizing conditions, quite rapidly, and it will be suflicient direction to state that the maximum oxide coating should be obtained.

It is also practical to impose upon the hot rolled and ickled sheets an artificial oxide coating. T iis is best done either by forming a water suspension of iron oxide, and s raying it upon the sheets, or passing the s eets between rolls which are supplied with a coatin of water sus ended oxide. A fairly thick coating shou d be applied, which covers the surfaces of the sheets fully.

I do not wish to limit myself to the three methods noted of coating silicon steel sheets with oxide, as any method of surfacing the sheets with the oxide will serve my purpose.

In the artificial coatings I have found that a grade of ferric oxide containing up to 90 per cent of the oxide, together with other metallic oxides, such as those of silicon, aluminum and calcium, will serve effectively, althoughmany other grades have been employed with satisfactory results, some of which have as low as 60% ferric oxide. Thus, by reference to iron oxide in the claims that follow, I wish to include substances which contain sufficient proportion of ferric oxide to take their character generally therefrom.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A rocess for treating silicon steel sheets w ich comprises forming upon the said sheets a coatin of iron oxide assemblin said sheets into pac s, and afterward annea ing said sheets under reducing conditions. 2. A process for treatin silicon steel sheets which comprises heating said sheets under oxidizing conditions to around 1200 F. so as to form a coating of oxide theeron, cooling said sheets, assembling them in piles, and afterwards annealing them under reducing conditions.

CARL E. GIFFORD. 

